This invention relates to infrared absorbing compounds and to imageable elements that contain these compounds. More particularly, this invention relates to infrared absorbing compounds in which both the anion and the cation absorb near infrared and/or infrared radiation.
In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically, comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. If, after imaging, the imaged regions of the imageable layer are removed in the developing process revealing the underlying hydrophilic surface of the substrate, the precursor is positive-working. Conversely, if the unimaged regions are removed by the developing process, the precursor is negative-working. In each instance, the regions of the imageable layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital imaging of offset printing plates, which obviates the need for imaging through a negative, is becoming increasingly important in the printing industry. Negative-working, alkaline developable imageable elements that comprise compounds that form an acid on thermal imaging have been developed for use with infrared lasers. For example, Haley, U.S. Pat. No. 5,372,907, discloses a radiation-sensitive composition that is sensitive to both ultraviolet and infrared radiation. The composition comprises (1) a resole resin, (2) a novolac resin, (3) an acid generator, and (4) an infrared absorbing compound.
Although acid generation is relatively efficient in systems in which the acid is generated by imaging with ultraviolet radiation, it is less efficient in which the acid is generated by imaging with near infrared and infrared radiation (700 nm to 3000 nm). In addition, the infrared absorbing compounds typically have counter ions that contain halogen and/or heavy metals, such as arsenic or antimony, materials that can cause environmental contamination. When the imaged elements are heated after imaging, highly corrosive acids, such as hydrochloric acid, can be formed and released into the atmosphere. Thus, a need exists for a more efficient method for forming images by direct digital imaging with infrared radiation that does not require an infrared absorbing compound that contains halogen or a heavy metal.
In one aspect, the invention is an infrared absorbing compound. The infrared absorbing compound has the structure: 
in which:
Y1 and Y2 are each independently hydrogen, halo, alkyl, diphenylamino, or phenylthio;
R1, R2, R3, and R4 are each independently hydrogen, methyl, or SO3xe2x88x92, with the proviso that two of R1, R2, R3, and R4 are SO3xe2x88x92;
R5 and R6 are each independently an alkyl group;
Z1, Z2, Z4, and Z5 are each independently a benzo group or a naphtho group;
Z3 and Z6 are each independently two hydrogen atoms, a cyclohexene residue, or a cyclopentene residue;
X1, X2, X3, and X4 are each independently S, O, NH, CH2, or C(CH3)2; and
n1 and n2 are each independently 0 to 4.
In another aspect, the invention is an imageable element comprising the infrared absorbing compound. In another aspect, the invention is a method for forming an image by imaging the imageable element with infrared radiation and developing the imaged imageable element.
When used as photothermal conversion materials in imageable elements, these infrared absorbing compounds provide an efficient method for forming images by direct digital imaging with near infrared and/or infrared radiation. Because each compound contains two chromaphores, they are efficient absorbers of a wide range of infrared and near infrared radiation. They improve both the dot stability and sensitivity of imageable elements. In addition, they are halogen free and contain no heavy metals such as arsenic or antimony.